That's really weird.
I've tried different browsers, and get the same problem.
As far as i see, in general search you can enter any airport code, and the search engine recognizes it.
However, when i force the engine to a specific airline (NH in this case), it blocks the airports that the airline does not fly to. For instance, try to enter TXL, HAM, CGN, whatever. The search engine will not recognize the airport. Same with DUS. Works fine with MUC and FRA and BRU, however.
Do you think it's a browser issue indeed, or am i doing something totally wrong?